Novel Organo-Noble-Gas Hydrides

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University of Helsinki Faculty of Science Department of Chemisty

Novel Organo-Noble-Gas Hydrides

Hanna Tanskanen

ACADEMIC DISSERTATION

To be presented, with the permission of the Faculty Science of the University of Helsinki, for public criticism in the Main lecture hall A110 of the Department of Chemistry (A. I. Virtasen aukio 1, Helsinki) on September 26^th 2009, at 12 noon.

Helsinki 2009

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Supervisors

Professor Markku Räsänen Department of Chemistry

University of Helsinki Docent Leonid Khriachtchev

Department of Chemistry University of Helsinki

Opponent Professor Zofia Mielke

Faculty of Chemistry University of Wroclaw

Reviewers

Professor Helge Lemmetyinen

Department of Chemistry and Bioengineering Tampere University of Technology

Doctor Austin Barnes School of Sciences – Chemistry

University of Salford

ISBN 978-952-10-5680-2 (paperback) ISBN 978-952-10-5681-9 (PDF) Helsinki University Printing House Helsinki 2009

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Abstract

Noble gases are mostly known as inert monatomic gases due to their limited reactivity with other elements. However, the first predictions of noble-gas compounds were suggested by Kossel in 1916, by von Antropoff in 1924, and by Pauling in 1930. It took many decades until the first noble-gas compound, XePtF6, was synthesized by Neil Bartlett in 1962. This was followed by gradual development of the field and many noble- gas compounds have been prepared. In 1995, a family of noble-gas hydride molecules was discovered at the University of Helsinki. These molecules have the general formula of HNgY, where H is a hydrogen atom, Ng is a noble-gas atom (Ar, Kr, or Xe), and Y is an electronegative fragment. The first molecular species made include HXeI, HXeBr, HXeCl, HKrCl and HXeH. Nowadays the total number of prepared HNgY molecules is 23 – including both inorganic and organic compounds. The first and only neutral ground-state argon compound, HArF, was synthetized in 2000. Helium and neon are the only elements in the periodic table that do not form neutral, ground-state molecules.

In this Thesis, experimental preparation of eight novel xenon- and krypton-containing organo-noble-gas hydrides made from acetylene (HCCH), diacetylene (HCCCCH) and cyanoacetylene (HCCCN) are presented. These novel species include the first organic krypton compound, HKrCCH, as well as the first noble-gas hydride molecule containing two Xe atoms, HXeCCXeH. Other new compounds are HXeCCH, HXeCC, HXeC4H, HKrC4H, HXeC3N, and HKrC3N. These molecules are prepared in noble-gas matrices (krypton or xenon) using ultraviolet photolysis of the precursor molecule and thermal mobilization of the photogenerated H atoms. The molecules were identified using infrared spectroscopy andab initio calculations.

The formation mechanisms of the organo-noble-gas molecules are studied and discussed in this context. The focus is to evidence experimentally the neutral formation mechanisms of HNgY molecules upon global mobility of H atoms. The formation of HXeCCXeH from another noble-gas compound (HXeCC) is demonstrated and discussed.

Interactions with the surrounding matrix and molecular complexes of the HXeCCH molecule are studied. HXeCCH was prepared in argon and krypton solids in addition to a Xe matrix. The weak HXeCCH CO2 complex is prepared and identified. Preparation of the HXeCCH CO2 complex demonstrates an advanced approach to studies of HNgY complexes where the precursor complex (HCCH CO2) is obtained using photolysis of a larger molecule (propiolic acid).

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Acknowledgements

This research was carried out in the Laboratory of Physical Chemistry at the University of Helsinki during years 2003-2007.

I would like to express my gratitude to our group leader and my supervisor Professor Markku Räsänen for introducing me to the field of matrix isolation and giving me an opportunity to work in his group. My second supervisor, Dr. Leonid Khriachtchev is greatly acknowledged and appreciated for his encouragement and invaluable guidance during the research. I am very grateful to Professor Jan Lundell for guidance in the field of computational chemistry and for many useful scientific and non-scientific discussions.

I would like to express my gratitude towards co-authors in Papers I-IX for their contribution to the research. I would also like to acknowledge the reviewers of this Thesis, Dr. Austin Barnes and Professor Helge Lemmetyinen, for their work. Head of Laboratory, Professor Lauri Halonen, and all other present and former members of the Laboratory of Physical Chemistry are thanked for making the laboratory a pleasant working environment. Especially I would like to thank my colleagues, Dr. Antti Lignell and Dr.

Susanna Pehkonen for sharing the days in the lab and making the days brighter. I want also warmly thank Professor Maija Aksela for her encouragement and support.

Magnus Ehrnrooth Foundation, University of Helsinki, Emil Aaltonen Foundation, and Assosiation of Finnish Chemical Societies are thanked for the financial support during this research.

Finally, I thank my family. My mom and my sisters are thanked for their continuous support and encouragement. My beloved Arto and our son Aarni are thanked for their love, support and patience during the work.

Tusen takk!

Hanna

Tromsø, June 2009

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List of original publications

This Thesis is based on the following publications which are referred to by the Roman numerals I-IX:

I L. Khriachtchev, H. Tanskanen, J. Lundell, M. Pettersson, H. Kiljunen, and M. Räsänen,

Fluorine-free Organoxenon Chemistry: HXeCCH, HXeCC, and HXeCCXeH J. Am. Chem. Soc.125, 4696 (2003).

II L. Khriachtchev, H. Tanskanen, A. Cohen, R. B. Gerber, J. Lundell, M.

Pettersson, H. Kiljunen, and M. Räsänen, A Gate to Organokrypton Chemistry: HKrCCH J. Am. Chem. Soc.125, 6876 (2003).

III H. Tanskanen, L. Khriachtchev, J. Lundell, H. Kiljunen, and M. Räsänen, Chemical Compounds Formed from Diacetylene and Rare-Gas Atoms:

HKrC4H and HXeC4H

J. Am. Chem. Soc.125, 16361 (2003).

IV H. Tanskanen, L. Khriachtchev, J. Lundell, and M. Räsänen,

Organo-noble-gas hydride compounds HKrCCH, HXeCCH, HXeCC, and HXeCCXeH: Formation mechanisms and effect of ¹³C isotope substitution on the vibrational properties

J. Chem. Phys.121, 8291 (2004).

V H. Tanskanen, L. Khriachtchev, J. Lundell, and M. Räsänen, HXeCCH in Ar and Kr matrices

J. Chem. Phys. 125, 074501 (2006).

VI L. Khriachtchev, A. Lignell, H. Tanskanen, J. Lundell, H. Kiljunen, and M.

Räsänen,

Insertion of Noble Gas Atoms into Cyanoacetylene: An ab initio and Matrix Isolation Study

J. Phys. Chem. A110, 11876 (2006).

VII L. Khriachtchev, H. Tanskanen, and M. Räsänen

Selective and reversible control of a chemical reaction with narrow-band infrared radiation: HXeCC radical in solid xenon

J. Chem. Phys.124, 181101 (2006).

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VIII H. Tanskanen, S. Johansson, A. Lignell, L. Khriachtchev, and M. Räsänen Matrix-isolation and ab initio study of the HXeCCH···CO₂complex

J. Chem. Phys.127, 154313 (2007).

IX H. Tanskanen, L. Khriachtchev, A. Lignell, M. Räsänen, S. Johansson, I.

Khyzhniy, and E. Savchenko,

Formation of noble-gas hydrides and decay of solvated protons revisited:

diffusion-controlled reactions and hydrogen atom losses in solid noble gases Phys. Chem. Chem. Phys.10, 692 (2008).

The candidate, Hanna Tanskanen, has done most of the experimental work in publications I-IX where she prepared samples, measured FTIR spectra, and performed excimer laser photolysis. She has also participated in the Ar⁺ laser and OPO photolysis together with Dr.

Leonid Khriachtchev. The candidate has also taken part in the syntheses of diacetylene, cyanoacetylene, and deuterated acetylene in guidance of M.Sc. Harri Kiljunen. She has performed all calculations in publication V and together with Dr. Antti Lignell and M.Sc.

Susanna Johansson in publication VIII. Other calculations were provided by Prof. Jan Lundell (Papers I-IV, VI) and Dr. Antti Lignell (Paper VI). The candidate has been the corresponding author in Papers III-V, VIII and IX and the coauthor in Papers I, II, VI, and VII.

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1 Introduction

Noble gases are mostly known as inert monatomic gases, and seldom chemical reactivity is connected with them. Nevertheless, a number of ground-state noble-gas compounds of radon, xenon, krypton and argon are known. Only for helium or neon no neutral chemical compounds have been found so far.^1-9

The noble-gas atoms were discovered around the turn of 20^th century.^10,11 The first predictions of noble-gas compounds were suggested in 1916 by Kossel and in 1924 by von Antropoff,^12-14 but only in the 1960’s the finding of the first noble-gas compound led to rich experimental xenon and krypton chemistry.¹⁵ Novel noble-gas compounds (noble- gas hydrides) were found by using matrix isolation techniques in 1990’s.^2,7,16 One of the recent matrix isolation achievements includes the finding of the first argon containing molecule.^17,18 The general description of noble-gas chemistry can be found elsewhere.¹⁹

This introduction mainly focuses on the history of neutral noble-gas compounds. It should be noted that ions, excited states and weak complexes of noble gases have been also studied extensively, broadening the field of noble-gas chemistry.²⁰

1.1 A glance to the history of noble-gas compounds

All noble gases were discovered before the 20^th century and they were considered chemically inert for quite a long time.^10,11Kossel, von Antropoff and Pauling suggested that noble-gas atoms could form compounds with halogens.^12-14,21 However, attempts were unsuccessful at that time.^22-24 The breakthrough in noble-gas chemistry was achieved in 1962 when Neil Bartlett reported a successful synthesis of the first noble-gas compound, xenon hexafluoroplatinate Xe⁺[PtF6] , which was later found to be a mixture of XeF⁺PtF6

and XeF⁺PtF11 .^15,25,26 Almost simultaneously with Bartlett’s work, Hoppe et al.

synthesized XeF2 and Claassen et al. reported XeF4.^27,28 After these discoveries, several compounds where xenon is bound to fluorine and oxygen have been made (for example, XeF6, XeOF4 and XeO2F2).^29-34 The fluorine and oxygen chemistry of xenon has been extensively reviewed by several authors.^35-38 The theory of chemical binding of xenon is interesting and it has been discussed thoroughly by Coulson.³⁹

Even though most of the known noble-gas compounds possess Xe-O or Xe-F bonds, there are examples where xenon is bound to other elements. The first Xe-Cl bond in XeCl2

was reported by Nelson and Pimentel in 1967, and this molecule was prepared in a low- temperature xenon matrix.⁴⁰ The first experimental example of the Xe-B bond in FXeBF2

was demonstrated by Goetschel and Loos in 1972.⁴¹ The first Xe-N bond was reported in 1974 with the discovery of FXeN(SO2F)2.⁴² In 1979, the formation of Xe(CF3)2 was reported presenting the first Xe-C bond, but it has not been spectroscopically characterized.⁴³ Besides this species, several other compounds containing a Xe-C bond have been synthesized since 1989 like fluorophenylxenon (II) fluoroborate ([C6H5Xe][B(C6H5)3F]) and a number of other organoxenonium compounds.^44-47 The first bulk compound containing a noble-gas-noble-metal bond, [AuXe42+

][Sb2F11 ]2, was

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reported by Seidel and Seppelt in 2000.⁴⁸ An extensive review of chemistry of xenon derivatives was written by Brelet al.³

Krypton chemistry was launched in 1963, one year later than the discovery of the first xenon molecule. Turner and Pimentel reported the first krypton compound, KrF2.⁴⁹ The impotance of KrF2 is emphasized by the fact that practically all following synthetic krypton chemistry is based on this species.⁵ In 1988, Schrobilgen prepared (HCNKrF)⁺(AsF6) , which represents the first Kr-N bond.⁵⁰ One year later, Schrobilgen and Sanders synthesized Kr(OTeF5)2 which was the first example of a Kr-O bond.⁵¹

In comparison with xenon, organic compounds containing krypton are very exceptional,⁵ and no neutral Kr-containing organic molecule was known before HKrCCH.^5,52,II The other known species with a Kr-C bond are the CH3Kr⁺ cation,⁵³ the inorganic HKrCN molecule,⁵⁴ and the organo-noble-gas species HKrC4H and HKrC3N prepared later.^III,VI

Recent research on chemical reactivity of noble gases has benefited from studies in solid noble gases at low temperatures. The matrix isolation technique was originally developed for studies of reactive and unstable species.^55,56 Most of the matrix isolation studies have been performed in solid argon and neon (see more information on matrix isolation in Experimental section 2.1.1). However, krypton and xenon matrices appear to react with many isolated species and this can be used for synthetic purposes.⁵⁷Examples of such reactivity at low temperatures is evidenced by noble-gas containing molecules like KrF2,⁴⁹ XeCl240

and XeClF.⁵⁸ An important development in noble-gas chemistry taking advantage of the reactivity of the surrounding was the preparation of noble-gas hydrides HNgY, where Ng is a noble-gas atom and Y is an electronegative fragment. This family of molecules was experimentally introduced at the University of Helsinki in 1995 and the experiments employed the matrix isolation technique.^59,60 The first molecular species were HXeI, HXeBr, HXeCl, HKrCl and HXeH. Nowadays the total number of prepared HNgY molecules is 23 – including the first neutral ground-state argon compound, HArF.^17,18

1.2. Noble-gas hydrides

The first noble-gas hydride molecules, HNgY, were found more than ten years ago and at present over twenty HNgY species are known. The HNgY molecules are high-energy metastable species with respect to the HY + Ng asymptote, and they are formed from the neutral H + Ng + Y fragments.⁶¹ Most of the significant studies on these molecules have been performed in noble-gas matrices. However, HXeI, HXeH, HXeCl and HXeCCH have also been reported in the gas-phase Xe clusters.^62-64 For the HNgY molecules, the synthetic procedure in matrices is quite straightforward and has produced many compounds exhibiting previously unknown chemical bonds such as Xe-S, Xe-I, Kr-Cl, Kr-C, H-Ar, and Ar-F.17,59,60,65

Additionally, a novel group of halogen-free organo-noble- gas molecules has been introduced, that is the main subject at the research presented in this Thesis.^I-VI The HNgY molecules are generally prepared at low temperatures from a suitable hydrogen-containing precursor (HY), which is dissociated by UV photolysis. The

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photolysis of HY produces hydrogen atoms and electronegative⁽¹⁾ fragments (Y). Next, H atoms are thermally mobilized at 20-45 K depending on the matrix to promote their reactions with neutral Ng + Y centers.^7,16 There are examples of HNgY molecules (HArF, HKrCl, HXeNCO) which also form directly during photodissociation.^17,66,67

HNgY molecules have been attributed to possess a strong (HNg)⁺Y ion-pair character, and their large dipole moments make them attractive systems to investigate electrostatic interactions with surrounding.^16,68,69 HNgY molecules can be easily detected by IR spectroscopy due to their very strong H-Ng stretching absorption with characteristic spectral positions.^7,16 The experiments show that the vibrational properties of the HNgY molecules are sensitive to the local environment.^18,70

The structural and energetic properties of the HNgY molecules are known only from quantum chemical computations. Electronic structure methods such as perturbation theory (mostly MP2, but MP3 and MP4 as well) and coupled cluster [CCSD, CCSD(T)]

calculations have been employed. From the computational point of view, the noble-gas compounds are challenging due to a large number of electrons. The ab initio calculations provide information on equilibrium structures and computational vibrational spectra, energetics and stability, and also on partial charges of HNgY molecules. The studies of HNgY molecules provide a good example where the computational and experimental approaches are combined successfully.

1.2.1 Nature of bonding and energetics of the HNgY molecules

The origin of bonding in HNgY molecules can be understood based on the model where both ionic (HNg⁺Y ) and neutral (HNgY) bonds contribute.^2,7,16,59This type of bonding was first suggested in a computational study by Last and George in 1988 when semiempirical DIIS (Diatomics In Ionic System) method predicted the existence of HXe⁺Cl .⁷¹The HNgY molecules have a strong ionic nature in their equilibrium structure and the molecule may be viewed as an ion-pair consisting of HNg⁺ and Y . This model means that the bonding in the H-Ng fragment is mostly covalent and the Ng-Y bond is mainly ionic. When the HNgY molecules are stretched along the molecular axis (H-Ng-Y) the ionic potential surface describing the motion is crossed by a repulsive electronic surface leading to the neutral fragments H + Ng + Y. This has been shown for HXeI and HXeCC,^61,VII and theoretically for HArF and recently for HXeCCH.^72,73 The dissociation limit of the HNgY molecules corresponds to the neutral fragments due to the avoided

(1)Electronegative Describing elements that tend to gain electrons and form negative ions. The halogens are typical electronegative elements. There are various ways of assigning values for the electronegativity of an element. Mulliken electronegativities are calculated from E= (I+A)/2, where I is ionization potential and A is electron affinity. More commonly, Pauling electronegativies are used. These are based on bond dissociation energies using a scale in which fluorine, the most electronegative element, has a value 4.

A Dictionary of Chemistry, edited by J. Daintith, 3^rd edition (Oxford University Press, Oxford, New York , 1996)

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crossing between the neutral and ionic potential energy surfaces. For HArF, these contributions have been verified by MRCI (Multireference configuration interaction) yielding an energy barrier of 0.33 eV for HArF dissociation to H + Ar + F fragments.⁷² In a qualitative literature model, the ionization potential (IP) of Ng, electron affinity (A)⁽²⁾ of Y and the dissociation energies of NgH⁺ can be used to construct the energetics between ionic and neutral dissociation limits.² Low ionization potentials for Ng and high electron affinities for Y are favourable to the stability of HNgY molecules enhancing the HNg⁺Y ion-pair Coulombic attraction.² Two factors make this electronic structure energetically favorable: the NgH⁺ ions are strongly bound^74-77 and the Y fragments in all the found HNgY molecules have high electron affinities. Also, the small Y fragments stabilize HNgY more than the larger ones, because they allow a closer approach of the HNg⁺ fragment, which leads to stronger Coulombic attraction between the charged species. It should be remembered, that the ionic model is simplified and other electronic configurations contribute also to the wavefunction describing the HNgY molecule. Other important resonance structures are neutral HNgY and ionic H X⁺Y.^2,7,16 For HArF, Avramopoulos et al. calculated the resonance structures of H-Ar⁺F H Ar²⁺F H⁺ArF .⁷⁸ Berski et al. have studied bonding and delocalization of electron density in these chemical systems using topological analysis of the electron localization function (ELF).⁷⁹ The calculations reveal that all molecules included in their study (HXeCN, HKrCN, HXeSH and HXeOH) are charge-transfer systems with the approximate formulas [HKr]^+0.65[CN] ^0.65, [HXe]^+0.66[CN] ^0.66, [HXe]^+0.45[SH] ^0.45, and [HXe]^+0.57[OH] ^0.57. It was shown that the Ng-C, Ng-N, Xe-S, and Xe-O bonds are of the unshared electron type and are mainly of electrostatic origin, i.e. the interaction is mainly ionic but with non- neglible fraction of a covalent character. This conclusion is in agreement with the simple model where both neutral and ionic potential surfaces contribute to the HNgY molecule.

Recently, the bonding of HXeCCH was studied.^1,73 In this molecule, the acetylenic group is strongly electronegative leading to the (H-Xe)⁺(CCH) ionic interaction. The structure, the NBO partial charges and the bond orders are presented in Fig. 1. The large partial charge +0.83e on Xe indicates a powerful electron-withdrawing effect of CCH.

Between (H-Xe)⁺and CCH , the bond order is very small (0.07) showing that the bond is mainly ionic without significant covalent contribution. The bond order value of 0.796 in (H-Xe)⁺ shows an essentially full two-electron covalent bond. The electron transferred from Xe to form CCH goes to an sp-type orbital in the bonding mechanism of HXeCCH and the highest occupied molecular orbital appears quite delocalized, mostly on the CCH group.^1,73

All HNgY molecules are highly metastable species. The decomposition process for HNgY HY + Ng is always strongly exoergic; however, the decomposition is prevented

(2)Electron affinitySymbol A. The energy change occurring when an atom or molecule gains an electron to form a negative ion. For an atom or molecule X, it is the energy realesed for the electron-attachment reaction X(g) + e X (g). Often this is measured in electronvolts.

A Dictionary of Chemistry, edited by J. Daintith, 3^rd edition (Oxford University Press, Oxford, New York , 1996)

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by high barriers.^1,72,80-82 The other decomposition channel HNgY H + Ng + Y is endothermic for all experimentally prepared molecules. For HXeCCH (see Fig. 2), the barrier for the HXeCCH HCCH + Xe reaction is over 2 eV and the barrier for the HXeCCH H + Xe + CCH is 0.98 eV.^1,73,83 However, the backward reaction (H + Xe + CCH HXeCCH) has a small barrier of 0.022 eV implying the formation of the molecule by this process is very efficient.⁷³

Figure 1 Bonding in HXeCCH. The bond lengths are in Å and bond orders are in parentheses. The figure is reproduced from Ref. 1.

Figure 2 Energetic stability of HXeCCH via (a) bending and (b) stretching coordinate. The figure is reproduced from Ref. 1.

Computing the H + Ng + Y dissociation path is extremely difficult due to involvement of several electronic configurations.72,73,80,81,84-86

All experimentally observed HNgY molecules are computationally below the neutral H + Ng + Y dissociation limit and the existence of this barrier is not the determining factor for their stability. The DFT and MP2 calculations are able to give reasonable estimates especially for the bending barrier, but for the H+ Ng + Y stretching coordinate, the MP2 method can give spurious results and more sophisticated methods are required.^81,87,88 The MP2 method has been widely used in computing energies of HNgY molecules and many of the theoretical predictions were experimentally realized. However, the MP2 is often inaccurate with respect to the relative energies of the HNgY molecule and the H + Ng + Y reagents, especially for larger open- shell species.⁸⁷

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1.3 Organo-noble-gas chemistry

In organic chemistry of noble-gas compounds, XeF227

and XeF428

and their related cations [XeF]⁺ and [XeF₃]⁺, have been widely used in synthesis.^3,4 The [FXe⁺] cation is the basis for preparation of new xenon-carbon bonds due to its strong oxidation potential.

Additionally, organoxenonium salts offer a promising route to the Xe-C compounds due to their lower oxidation potential in comparison with the [FXe]⁺ cation.⁴ The stability and reactivity of organoxenonium salts [RXe]⁺[Y] depends on the nature of the organic group R and counteranion [Y ].⁴ At the present time, various types of organoxenon compounds are known: mononuclear xenonium(II) salts [RXe]⁺[Y] (where R = aryl, polyfluoroalkenyl, alkynyl and Y = a counteranion), binuclear xenonium(II) salts [(C₆F₅Xe)₂Z]⁺[AsF₆] (where Z = F or Cl), arylxenonium(IV) salt [C₆F₅XeF₂]⁺[BF₄] , covalent xenon(II) compounds C6F5XeZ [where Z = F, Cl, CN or OC(O)C6F5], symmetric and asymmetrical diorganoxenon(II) compounds, R₂Xe (where R = C₆F₅ or 2,4,6-C₆H₂F₃), and RXeR’ (where R = C6F5, and R’ = 2,4,6-C6H2F3), respectively.⁴ Recently, Frohn and Bardin have reported the preparation of an alkynylxenon(II) compound [CF3 CXe][BF4].⁸⁹ Recent reviews on organoxenon chemistry have been written by Brel et al. and by Frohn and Bardin.^3,4

In 2000, it was predicted that Xe could be inserted into carboxylic acids, and chemical binding of xenon to proteins was hypothesized.^90,91 However, no experimental verification of these results has been reported. In 2002, Lundellet al. presented the first prediction on fluorine-free alkynylxenon derivatives.⁸³ They reported computational predictions of HXeCCH and other organic molecules such as Xe-insertion compounds of benzene and phenol. These calculations also described molecules with more than one xenon atom such as HXeCCXeH and HXeCCXeCCXeH. It was suggested that such xenon-insertion molecules could form a new class of possible precursors and intermediates for synthetic and organoelement chemistry.⁸³ These predictions have been realized for HXeCCH and HXeCCXeH which were prepared experimentally briefly after the computational prediction.^I,92 The existence of the larger systems quoted by Lundell et al. and shown in Fig. 3 remains still an open question. Feldman and coworkers have reported failed attemp to insert Xe into benzene.⁹³

Figure 3 Computationally predicted and characterized compounds with Xe inserted to acetylene, benzene and phenol. The structural models of molecules are adapted from Ref. 83.

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1.4 Aim of this study

In this Thesis, experimental preparation of eight novel xenon- and krypton-containing organo-noble-gas hydrides made from acetylene (HCCH), diacetylene (HCCCCH) and cyanoacetylene (HCCCN) are presented.

The main focus of this Thesis at the first stage was to test experimentally the hypothesis of fluorine-free organo-noble-gas compounds. The first experimental target was the stoichiometrically simplest species, HXeCCH in solid xenon^I and HKrCCH in solid krypton.^II,IV Once it was clear that HXeCCH and HKrCCH could be made, the chemistry of HXeC_2nH and HKrC_2nH compounds were studied further. A promise for larger molecules originates from the increasing electron affinies of longer carbon chains.

The electron affinities (EA) are 2.956 eV for CCH, 3.558 eV for C₄H, 3.809 eV for C₆H, and 3.996 eV for C8H.⁹⁴ Therefore, insertion of Ng atoms into HC2nH (where n = 1, 2, 3,…) is a realistic idea providing an experimental test for the simple model of bonding described in Chapter 1.2.1. The experiments with diacetylene were a step to increase the electronegativity of the Y fragment. The HXeC₄H and HKrC₄H seem to be more strongly bound than HXeC2H and HKrC2H.Î-III It was found that the most of the electron affinity in carbon chain was localized at the carbon end of the radical.ÎII The preparation of HArC₄H was tried because EA of C4H radical is larger than that of a F atom (~3.4 eV). However, this compound did not form in experiment.ÎII The experiments with HCCCN were started based on the high electron affinity of CCCN (4.59 eV)⁹⁵ which gave a possibility to make an argon compound. Unfortunately, no argon-containing organo-noble-gas compound has been prepared so far.

Additionally, the formation mechanisms of organo-noble-gas molecules and the matrix effects are studied and discussed in this context. The focus is to evidence experimentally the neutral formation mechanisms of HNgY molecules upon global mobility of H atoms.

The formation of HXeCCXeH from another noble-gas compound (HXeCC) is demonstrated and discussed. Finally, the weak complex of HXeCCH CO₂ is prepared and identified.

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2 Methods

2.1 Experimental methods

2.1.1 Matrix isolation

The matrix isolation technique was originally introduced in the 1950’s as a method to investigate reactive intermediates and unstable species, for example radicals.^55,56 In this approach, the studied species are trapped at low temperatures (typically between 4 and 80 K) in an inert solid matrix. These conditions considerably increase the lifetime of the trapped unstable species and decrease its interaction with other atomic or molecular species. The matrix isolation technique has been widely and successfully used for decades with different detection techniques including infrared (IR), laser-induced fluorescence (LIF), Raman, and electron paramagnetic resonance (EPR) spectroscopy.^96,97 Most of the matrix isolation studies nowadays utilises noble gases as matrix materials, mainly argon and neon, but xenon and krypton solids have been used as well. The noble-gas solids do not absorb in the IR region and for most purposes they are chemically inert.⁹⁶ Nitrogen, hydrogen or oxygen as well as other molecular solids can also be used as a matrix material.

2.1.2 Experimental procedure

In this research, acetylene (HCCH), diacetylene (HCCCCH), cyanoacetylene (HCCCN) and propiolic acid (HCCCOOH) have been used as precursor molecules. The diacetylene and cyanoacetylene molecules were synthetized by M.Sc. Harri Kiljunen.

Gaseous mixtures of the studied molecule and noble gas were prepared in various proportions in a glass bulb. The typical absorber to matrix (A/M) ratios varied between 1:300 and 1:2000. The mixtures were deposited onto a CsI window cooled with a closed- cycle helium cryostat (DE-202, ADP) at typically 10-30 K, depending on the matrix gas.

The selection of the deposition temperature was always a compromise between the monomeric absorber and the optical matrix properties. A typical matrix thickness was

~100 m. After deposition of the gaseous sample onto the cold window, the matrix is cooled to the lowest operation temperature (7-9 K). The photolysis of the matrix was carried out at this temperature. Then the photolyzed matrix was annealed up to ~30 K (krypton) or 40-45 K (xenon) to mobilize hydrogen atoms.^98-101 After annealing the matrix spectra were measured at 7-9 K.

The infrared (IR) absorption spectra (4000-400 cm ¹) were measured with a Nicolet 60SX Fourier-transform (FTIR) spectrometer typically with 1 cm ¹ resolution coadding 200-1000 scans. Photolysis of the precursor molecules was performed mainly with an excimer laser (MPB, MSX-250) operating at 193 nm or with an optical parametric

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oscillator (OPO) at 250 nm. Other radiation sources were also used, for example 488 nm (Ar⁺) radiation was used to decompose some of the annealing-induced products. Detailed information of the sample preparation and experimental condition employed in these studies can be found in Papers I-IX.

2.2 Computational methods

In this section, only short introduction to ab initio methods is given. The detailed information on the methods, approximations, procedures and applicabilities can be found elsewhere.¹⁰²

The ab initio methods are based on solving the time-independent Schrödinger equation.

( , ) ( , )

H r R E r R (1)

In this equation, H is the Hamiltonian operator; E is the energy (eigenvalue) of the stationary state representing by the wavefunction (eigenfunction, ) in the electronic (r ) and nuclear (R) coordinates. It is impossible to solve Schrödinger equation (1) analytically for molecular systems and various approximations are needed to solve the equation. The Born-Oppenheimer approximation separates electronic and nuclear motions.

The Schrödinger equation is separated into one part which describes the electronic wavefunction for a fixed nuclear geometry, and another part for the nuclear wavefunction where the energy from the electronic wavefunction plays the role of potential energy.

Schrödinger equation is solved depending only on the electronic coordinates and handling the nuclear coordinates as parameters. Using the Born-Oppenheimer approximation provides the effective electronic energy of the molecule in static field of the nuclei.

( ; ) ^eff( ) ( ; )

H r R E R r R (2)

This approximation allows construction of potential energy surface (PES) which describes the changes of molecular electronic properties when the nuclear coordinates are changed.

The electronic Schrödinger equation has to be solved for large number of nuclear geometries until the PES is known.

However, even with the Born-Oppenheimer approximation the Schrödinger equation is difficult to solve for molecular systems containing many atoms. Another useful approximation is the Hartree-Fock approximation where the energy of the electron depends on the electric field generated by the nuclei and the other electrons. By using the Fock operator, the n-electron equation is separable into n one-electron equations. There, the electron-electron repulsion is treated as an electrostatic energy between one electron and the charge density created by all other electrons. The total wavefunctions are described as products of the one-electron wavefunctions and when electron spin is included they are called spin-orbitals. The antisymmetrized product of the total wavefunction is called a Slater determinant. The Hartree-Fock equation is solved by using the self-consistent field (SCF) method employing an approximation where wavefunctions

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are generated iteratively. However, the Hartree-Fock approximation does not take into account the Coulombic electron-electron correlation because each orbital is a solution of one-electron Schrödinger equation. For a better result, it is necessary to correct the motions of the electrons. One of the methods that include the effects of electron correlation is the perturbation theory where the total Hamiltonian operator is written as a sum of the Hartree-Fock Hamiltonian (H0) and the perturbation term (V)

(H0 V) E . (3)

The is the strength parameter. When the eigenfunctions and eigenenergies are expanded in a Taylor series and grouped to the order of the strength parameter, the linear combinations of different orders of perturbation are obtained. The Møller-Plesset perturbation method to the second order is referred for example as MP2 and coupled cluster method as CCSD(T).

The molecular orbitals are generated from a linear combination of atomic orbitals.

These functions constitute the basis set. The Gaussian-type functions are mostly used in general QM programs. The Gaussian primitivesgpare given as

( ) ⁿ ^m ^l ^r2

gp c x y z e (4) where the orbital exponent is a parameter describing the radial extent of the function, c is the normalizing constant, n,m, and l are the powers of x, y, and z coordinates.

In this work, ab initio calculations have been used to support the experimental work.

Theab initiocalculations were carried out with the GAUSSIAN98 (Revision A.11.4) and GAUSSIAN03 (Revision B.02) packages of computational codes.^103,104 The electron correlation methods were the Møller-Plesset second order (MP2) perturbation theory where all electrons were taken explicitly into correlation calculations (full). For HXeCC, the coupled cluster CCSD(T) method was employed. The standard split valence 6- 311++G(2d,2p) basis set was applied for H, C, N, Kr, and Ar atoms. For Xe atoms, an effective core potential (ECP) was used for economical reasons because of the large number of electrons. In ECP, the core electrons are modelled by a suitable function and only valence electrons are treated explicity.¹⁰² The relativistic pseudopotential by LaJohn et al. was employed for Xe and it is denoted as LJ18 throughout this work. This ECP include the d subshell in the valence space resulting in 18 valence electrons. The valence basis set combined with ECP was used in a decontracted form.¹⁰⁵ In some calculations, the effective core potential of LaJohn et al. (LJ18) was combined with Runeberg and Pyykkö’s valence space and employed for Xe atoms.¹⁰⁶ In Paper VI, the anharmonic computations on Kr-containing molecules were performed with the vibrational self- consistent field (VSCF) method and its extensions by correlations via the second-order perturbation theory.^107-110 All anharmonic calculations based on the VSCF method were performed with the GAMESS (version R4, 2004) electronic structure program.¹¹¹ The C, N, and H atoms were decribed by the cc-pVDZ all electron basis set, whereas for krypton the cc-pVDZ-PP basis set were used.¹¹²

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3 Results and discussion

3.1 Identification of new organo-noble-gas hydrides

This section focuses on the preparation and identification of novel organo-noble-gas compounds in experiments with various precursor molecules C2H2, C4H2, and HC3N in solid noble gas matrices.

The characterization of the novel noble-gas molecules is based on the isotope substitution experiments and on the computed IR spectra. A detailed discussion of the assignments of the organo-noble-gas molecules can be found in Papers I-VI.

The calculated spectra are in most cases in good agreement with the observed spectra.

All computationally strong bands are usually found in the experiments. The harmonic MP2 H-Ng stretching frequencies are overestimated, but this is typical in the case of the HNgY molecules.^2,7,16 This overestimation is mainly due to anharmonicity not being taken into account in the calculations. The MP2 method is used as a standard method for economical reasons but for the energy calculations more accurate methods should be used.

The HNgY molecules have characteristic properties that can be used in their identification. They are formed upon annealing of the photolyzed HY/Ng matrix at ~ 40 K in solid xenon and at 30 K in solid krypton. The H-Ng stretching absorption is the most intense and very characteristic for the HNgY molecules. The noble-gas molecules usually decompose easily upon irradiation by light due to excitation to repulsive states.2,7,113,I,II,III,VI

3.1.1 HXeCCH, HXeCC, HXeCCXeH, and HKrCCH

The infrared (IR) spectra of acetylene in Ng (Ng = xenon, krypton and argon, C2H2/Ng) matrices and in argon matrix doped with xenon (C2H2/Xe/Ar) are presented in Fig. 4.

Acetylene can be photolyzed in solid xenon with UV light (for example, 193 nm) producing isolated H atoms, C2H radicals,¹¹⁴ Xe-CC complexes,¹¹⁵ and XeHXe⁺ ions.¹¹³ The C2H radicals are further photolyzed to C2molecules.¹¹⁵ 250 nm photolysis is used in some experiments with C2H2/Xe. The 193 and 250 nm photolysis products are similar, but the C2/C2H ratio is larger upon irradiation at 250 nm than at 193 nm. The larger C2/C2H ratio leads upon annealing to more efficient formation of HXeCC radicals as compared with HXeCCH. In solid krypton, the most prominent photolysis products of acetylene are C2H radicals, KrHKr⁺ ions,¹¹⁶ as well as C2. C2 are IR-inactive in a Kr matrix. Some formation of C4 clusters¹¹⁷ was observed especially in samples with a higher initial acetylene concentration. The photodecomposition of acetylene is more efficient in solid xenon (typically ~50-80 %) than in solid krypton (~20-30 %) for similar light exposures at 193 nm (typically up to 5000 pulses with pulsed energy density of 20 mJ/cm²).^I,II,IV This is probably connected with the fact that UV photolysis in noble-gas solids is often self- limited due to rising absorbers.¹¹⁸ 193 nm photolysis of C2H2/Ar sample generates C2H radicals as the main product. In mixed matrices [C2H2/Xe/Ng (Ng = Kr or Ar)], the

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CCH Xe and Xe-CC complexes are visible upon 193 nm photolysis.¹¹⁹ The IR-active photolysis products in various solids are collected in Table I.

Figure 4 IR absorption spectra of acetylene in solid xenon, krypton and argon at 8 K. The lower trace shows the effect of Xe atoms on the acetylene absorption bands in an Ar matrix. The experimental absorptions of acetylene in various matrices can be found in Paper V.

Table I 193 nm photolysis products (absorption wavenumbers in cm¹) of acetylene in Ar, Kr and Xe matrices. The dominating photolysis products are highlighted in bold.

Ar Kr Xe

12

C¹²CH 1846.5 1842 1852

13

C¹³CH 1782 1791

12

C¹²C-Xe 1774.3 1770.5 1767

13

C¹³C-Xe 1699

12

C¹²CH-Xe 1842.5 1844.3

12

C4 1539.5 1536

13

C4 1480 1477

KrHKr⁺ 852

1008

XeHXe⁺ 730.5

842.5 953

3400 3300 3200

0.5 1.0 1.5 2.0

750 675

0 2 4

12C₂H₂/Xe/Ar

12C₂H₂/Ar

12C₂H₂/Xe

13C₂H₂/Kr

Absorbance

Wavenumber (cm^-1)

12C₂H₂/Kr

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Subsequent annealing of the photolyzed matrix at 30 K (Kr) and at 45 K (Xe) mobilizes H atoms¹⁰⁰ and leads to the formation of various noble-gas molecules with characteristic H-Ng stretching bands: HKrCCH in solid krypton, and HXeCCH, HXeCC, HXeCCXeH, and HXeH in solid xenon.^I,II, Vinyl radicals, C2H3, are also formed upon annealing.^120,121 Annealing of photolyzed C2H2/Xe/Kr (at 30 K) and C2H2/Xe/Ar (at 20 K) matrices produces vinyl radicals and HXeCCH in solid krypton (in addition to HKrCCH) and argon. The absorption wavenumbers are collected in Table II. The spectra of these molecules in C2H2/Xe, C2H2/Kr and C2H2/Xe/Kr matrices in the H-Ng stretching region are presented in Fig. 5. In experiments with C2H2/Ar, no noble-gas molecules are produced.

Table II Annealing products of (absorption wavenumbers in cm ¹) acetylene (C2H2/Ng and C2H2/Xe/Ng) samples in Ar, Kr and Xe matrices. The strongest absorptions are marked with (s).

Ar Kr Xe

HXeCC 1478.3(s)

1474.7 HXeCCH 1531.3 1518.7 1486.4 (s)

1517.4 1505.6 1480.7 1482.2

1479.9

HXeCCXeH 1305.8

1300.9 (s) 1294.3

HXeH 1180

1166

HKrCCH 1256.6

1249.6 1241.6 (s)

C2H3 1356.7 1353.2 1348.9

900.8 896.6 891

C4H 2055

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Figure 5 Annealing-induced formation of Xe- and Kr-containing noble-gas compounds in different matrices. The IR spectra are measured at 8 K. (a) Difference spectrum showing the result of annealing at 45 K of a photolyzed C2H2/Xe (~1/1000) matrix.

(b) Result of annealing at 30 K of a photolyzed C2H2/Kr (~1/2000) matrix. (c) Result of annealing at 30 K of a photolyzed C2H2/Xe/Kr (~1/2/1000) matrix. All samples were photolyzed using 193 nm (~ 1800-2400 pulses, 10-20 mJ/cm²) before annealing.

The absorption bands of vinyl radical are marked with asterisk.¹²¹

The assignment of HXeCCH in a Xe matrix is based on several facts. The amount of the absorber HXeCCH with the H-Xe stretching band 1486 cm ¹ correlates with the C2H concentration after photolysis, and its bands at 3273 and 626 cm ¹ fit well the H-C stretching and H-CC bending modes. The deuteration and ¹³C experiments fully support this assignment.^I,III The harmonic MP2 calculations agree reasonably with the experimental values. The assignment of HKrCCH in solid krypton is done similarly to HXeCCH, and the formation of HKrCCH needs C₂H radicals in the matrix.^IV Such compounds as HKrCC or HKrCCKrH were not found, highlighting the lower reactivity of krypton compared to xenon.

Formation of HXeCC is enhanced by longer 193 nm or 250 nm initial photolysis which produce larger amounts of C₂ molecules. C₂ is an electronegative fragment and H atoms can react with the Xe-C₂ center. The selective photodissociation of HXeCC leads to an increase of the Xe-C₂concentration^I,VII HXeCC is also very photolabile compared to HXeCCH, HXeH and HXeCCXeH molecules.^III The absorption bands of the H-Xe

1600 1400 1200 900 800

0.0 0.1 0.2

HOO

(c) (b) (a)

C2H

2

HKrCCH

*C2H

3

*

C₂H₂ HKrCCH

HXeCCH

KrHKr+ XeHXe+

* HXeH HXeCCXeH HXeCC

HXeCCH

Absorbance

Wavenumber (cm^-1)

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stretching mode of HXeCC and HXeCCH partially overlap with each other, and the absorptions of both molecules are doublets. By using the selective photolysis, it is possible to separate the absorptions of HXeCC and HXeCCH from each other.

The assigment of HXeCCXeH at ~1301 cm ¹ is based on ab initio calculations and deuteration experiments. The ab initio calculations show the stability of the species and predict a decrease (by ~140 cm ¹) of its H-Xe stretching frequency as compared to HXeCCH. This is in agreement with the measured difference of 177 cm ¹. The experiments with partially deuterated acetylene provide a conclusive method to identify HXeCCXeH. For HXeCCXeD, two absorptions corresponding to the H-Xe and D-Xe stretching modes are expected and these bands should be shifted from the corresponding bands belonging to HXeCCXeH and DXeCCXeD. The MP2 calculations predicted blue shifts for the H-Xe and D-Xe stretching absorptions (+40 and +24 cm ¹, respectively) of HXeCCXeD from the corresponding bands of HXeCCXeH and DXeCCXeD. These shifts are experimentally determined to be +41 and +27 cm ¹, respectively (see Fig. 6).^I

The experimental and calculated vibrational frequencies of HXeCCH, HXeCC, HXeCCXeH and HKrCCH are collected in Tables VI and VII (Chapter 3.2).^I,II,IV

Figure 6 Difference IR absorption spectra demontstrating results of photodissociation of HXeCCXeH isotopologues (a) without deuteration and (b) with deuteration (degree of deuteration ~70 %). The species were obtained by 250 nm photolysis (1.5x10⁴ pulses, ~2 mJ/cm²) of acetylene in solid xenon and annealing at 45 K.

1400 1350 1300 1050 1000 950 900 -0.005

0.000 0.005

(a)

(b)

HXeCCXeH

HXeCCXeD HXeCCXeD DXeCCXeD

Absorbance

Wavenumber (cm^-1)

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3.1.2 HXeC4H and HKrC4H

The IR absorption spectra of C4H2 and C4D2 in a Kr matrix are presented in Fig. 7 and the recorded spectra indicate mainly monomeric trapping of the precursor molecule. The diacetyle-d2 sample containes practically no diacetylene-d. Similar results were obtained for Xe matrices.

Figure 7 IR absorption spectra of diacetylene (~ 1:1000) in solid krypton at 8 K. Spectra for C4H2 and C4D2 are shown by the upper and lower traces, respectively. The absorptions of diacetylene are presented in Paper III.

Photolysis at various wavelengths (193, 235, 240 and 250 nm) was used. The photolysis products of diacetylene in noble-gas matrices are C4H radicals,¹²² carbon clusters such as C₄¹¹⁷ and C₈,¹¹⁷ and NgHNg⁺ (see Table III). The C₄H concentration achieves in the initial stage of photolysis reaches its maximum, and decreases upon longer photolysis. The C₄ concentration increases monotonically. It should be noted, that the KrHKr⁺ ions^116,123 were not detected after photolysis at 240 and 250 nm, whereas this species was seen after photolysis at 235 and 193 nm.

Table III The main photolysis products of diacetylene in Kr and Xe matrices.

Kr Xe

C4H 2055 2050 C4 1539.5 1536 C8 2065.5 2057 KrHKr⁺ 852

1008

XeHXe⁺ 730.5 842.5 953

3000 2000 1000

0.0 0.5 1.0

C₄D₂/Kr C4H

2/Kr

Absorbance

Wavenumber (cm^-1)

(25)

Thermal annealing of photolyzed C4H2/Ng matrices mobilizes the H atoms and they can react with the Ng + C4H centers, which yields new molecules. The strongest bands are assigned to the H-Ng stretching modes of the HKrC4H and HXeC4H molecules (see Table IV and Fig. 8). In a Xe matrix, the formation of HXeH is also observed.⁶⁰ At higher- temperature annealing (above 40 and 55 K in krypton and xenon solids, respectively), a decrease of some lower-frequency H-Ng stretching bands and an increase of the higher- frequency bands components were observed. In experiments with C4H2/Ar, there was no detection of similar absorptions of HArC4H.

Table IV Main annealing products (in cm ¹) of diacetylene in Kr and Xe matrices. The (h) mark refers to the absorption bands observed after higher-temperature annealing.

Kr Xe HKrC4H 1275.5 (H-Kr str.) 1290

1307.5 1317 (h)

HXeC4H 1503.5

(H-Xe str.) 1521.5 1532 1545 (h) 1558.5 (h)

HXeH 1180

1166

The identification of HNgC4H molecules is straightforward. The obtained IR spectra strongly depend on the matrix medium. The new species exhibit typical properties of HNgY species such as strong H-Ng stretching absorptions with extensive matrix-site splitting. The noble-gas molecules usually decompose easily upon irradiation by light due to excitation to the repulsive states and this behaviour was found for both HKrC4H and HXeC₄H molecules. By using this property, weaker absorptions belonging to these respective species were identified. All computationally strong absorptions (the H-Ng stretch, the H-C stretch, and the C-C-H and C-C-C bendings) were experimentally identified. The deuteration experiments also support the assignment and the calculated spectra are in agreement with experiment.

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Figure 8 Difference IR absorption spectra of HXeC4H in solid xenon. The spectra are measured at 8 K. Shown are the result of annealing at 45 K of the photolyzed sample (the upper trace), the result of annealing at 65 K (the middle trace), and the result of 250 nm irradiation of the annealed sample (the lower trace). The absorption band marked with asterisk (*) is not assigned here to HXeC4H molecule because this is not supported by the deuteration experiments and calculations.

3.1.3 HXeC3N and HKrC3N

The deposited HCCCN/Ng (Ng = Xe, Kr, and Ar) samples were quite monomeric with respect to cyanoacetylene. The IR absorption bands of the HCCCN precursor in argon, krypton and xenon solids can be found in Paper VI.

Similarly to our experiments with acetylene and diacetylene, photolysis and annealing of cyanoacetylene/Ng matrices evidence the formation of noble-gas hydrides.

Accordingly, the strongest IR absorption bands are found to be at 1492.1 and 1624.5 cm ¹ for Ng = Kr and Xe, respectively (see Fig. 9). Deuteration of cyanoacetylene leads to a proper down-shift of the H-Ng stretching frequency with the H/D ratios of 1.349 and 1.380 for Ng = Kr and Xe, respectively. The experiments with ¹⁵N/¹⁴N-substitution were also performed to increase the confience of the assignment. The annealing-induced vibrational bands are efficiently bleached by UV light, which is also characteristic for noble-gas hydrides. The band fine structure of the Kr-containing molecule, which is most probably due to the matrix-site effect, is very similar to the H-Kr stretching bands of HKrCCH and HKrC4H.^II,IV New absorptions were observed in three spectral regions (H- Ng, C N, and C-C stretching modes) that belong to the HNgC3N molecules (see Tables VI and VII). The observed bands are also the strongest in the computed spectrum. The structural assignment was not straightforward due to possible HNgCCNC isomers possessing similar vibrational properties. However, the computational spectra of

1600 1500 0.4

0.6 0.8

1200 1050 900 640 560 Wavenumber (cm^-1)

Absorbance

250 nm 65 K 45 K

C4

HXeC4H (H-Xe)

*

(C-C-H) (C-C-C)

HXeH

C4H

2

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HKrCCCN and HXeCCCN fit most closely the experimental data, which is the basis for the assignment. The identified species also have the H-Ng stretching absorptions at quite similar frequencies to the known HKrCN and HXeCN molecules, which is in agreement with the theoretical predictions.⁵⁴ No strong candidates for an Ar compound were found in the experimental data.

Figure 9 Spectra of (a) HKrCCCN and (b) HXeCCCN in the H-Ng stretching region. The spectra were obtained by 193 nm photolysis of cyanoacetylene in noble-gas matrices and annealing at 30 K (Kr) and 45 K (Xe). The insert in panel (b) shows the correlation of the main band at 1624.5 cm ¹ with the weaker bands at 896 (triangles) and 2233 (circles) cm ¹ obtained by using photodissociation at 250 nm. The data points shown in the insert correspond to a different duration of photodissociation.

1550 1500 1450 1400

0.02 0.03

1650 1600 1550 1500

0.00 0.01 0.02

0 2 4

0 1

(a) - Kr

Absorbance

(b) - Xe

Wavenumber (cm^-1)

I(896), I(2233)

I(1624)

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3.1.4 Experimental and computational properties of organo-noble-gas molecules

In this section, the properties of new organo-noble-gas molecules are compared. The focus is on comparison of the stability of the observed organo-noble-gas molecules. Various computational properties of the molecules are also presented.

The H-Ng (Ng = Kr or Xe) stretching absorptions of the molecules are presented in Fig. 10. The H-Ng stretching absorption for HNgC3N is at higher wavenumbers than HNgC4H and HNgCCH indicating that HNgC3N is more strongly bound than the two other ones. The trend is similar for both Xe and Kr containing molecules. The H-Ng bond lengths support this trend. The computed H-Xe bond for HXeC3N molecule is ~ 0.01 and 0.02 Å shorter than for HXeC4H and HXeCCH. The H-Kr bond length for HKrC3N is

~0.04 and 0.06 Å shorter than for HKrC4H and HKrCCH. The calculated equilibrium structures of the novel organo-noble-gas compounds are collected in Fig. 11.

Figure 10 (A) Spectra of xenon insertion molecules: (a) HXeCCH, HXeCC, and HXeCCXeH, (b) HXeC4H, and (c) HXeC3N in the H-Xe stretching region. (B) Spectra of krypton insertion molecules: (a) HKrCCH (b) HKrC4H, and (c) HKrC3N in the H-Kr stretching region. The matrices were first UV photolyzed and then annealed at 45 K (Xe) and 30 K (Kr). The spectra are measured at ~ 8 K. Broad spectral features originate from background fluctuations.

1500 1350

0.10 0.15

HC₃N C2H

3

HKrC3N

HKrC4H HOO

(c) (b) (a)

C2H

2

HKrCCH B

1600 1400

0.0 0.1 0.2

HXeCCXeH

HXeC3N

HXeC4H

(c) (b) (a)

HXeCC HXeCCH

Absorbance

Wavenumber (cm^-1) C4

A

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.

Figure 11 Ab initio structures of HNgCCH, HNgC4H, HNgC3N, HXeCC, and HXeCCXeH (Ng

= Xe or Kr). The bond lengths are in Å. For calculations, the MP2/LJ18(Xe), 6- 311++G(2d,2p) level was used. For HXeCC, the computational level is CCSD(T)/LJ18[Xe],6-311++G(2d,2p).

Novel Organo-Noble-Gas Hydrides